Decade-long galaxy survey releases final catalog

By:
Linda B. Glaser,
A&S CommunicationsWed, 05/30/2018

The last data release and final official survey paper from the major Arecibo Legacy Fast ALFA (ALFALFA) survey, led by Cornell astronomers, was published July 2 in Astrophysical Journal Supplement. The paper marks the conclusion of a vast project that required 4,400 hours of telescope time and resulted in more than 100 papers co-authored by faculty, staff and students at Cornell. The survey was the basis of 10 Cornell doctorates and an equal number by graduate students from other universities.

“The population of galaxies we’ve detected include the most isolated galaxies in the universe, the least clustered,” said Martha Haynes, co-principal investigator. “Studying these galaxies reveals how galaxies evolve in the absence of interactions with other galaxies or with the hot gas found in clusters of galaxies.”

Among ALFALFA’s discoveries are enigmatic objects such as the nearby faint dwarf Leo P and Leoncino, a galaxy with very little metal content. ALFALFA also identified what the researchers call an “intriguing population” of massive galaxies with unusually high percentages of their mass as cool hydrogen gas.

ALFALFA fulfilled one of its primary goals by discovering a small population of galaxies with dark matter and gas and almost no detectable starlight. Researchers say they don’t yet understand why these galaxies haven’t turned their abundant gas into stars.

Since it began in 2005, ALFALFA has detected radio emission from cool gas in more than 30,000 galaxies, producing the first comprehensive census of gas-bearing galaxies out to a distance of 800 million light years from our galaxy, the Milky Way, encompassing nearly one-sixth of the sky -- or some 7,000 square degrees. The survey was the first to use the Arecibo L-Band Feed Array (ALFA) radio camera on the 305 meter diameter radio telescope at Arecibo Observatory in Puerto Rico, focusing on the frequency range that includes a spectral line emitted by neutral atomic hydrogen, the most abundant element in the universe. Nearly every previous sky survey has been of optically, infrared- or X-ray-selected galaxies.

The importance of the hydrogen, explains co-principal investigator Riccardo Giovanelli, emeritus professor of astronomy, is that gas-rich systems of low mass that have not been able to convert their cosmic material into stars -- the so-called dark galaxies – are optically inert because they have few or no stars, but can be detected by their hydrogen signature.

The data from the survey are available on the ALFALFA website and will be incorporated into the NASA Extragalactic Database (NED-NASA). It includes individual spectra, not just a catalog of positions, so other researchers can re-analyze the spectra.

Receiver engineer Ganesh Rajagopalan and Professors Riccardo Giovanelli and Martha Haynes conducted the last night of observing for ALFALFA from the Space Sciences Building.

While the scientific discoveries of ALFALFA have been impressive, its impact on teaching and learning through the Undergraduate ALFALFA Team has been equally profound. The team is a consortium of 23 mostly undergraduate colleges around the country for which Haynes serves as science lead; Rebecca Koopmann, professor and chair of the Department of Physics and Astronomy, Union College, serves as principle investigator.

“ALFALFA wasn’t a big-grant, big-team, big-science project but rather something that a small, distributed group has done to undertake a major scientific contribution,” said Haynes, Goldwin Smith Professor of Astronomy. “It’s a legacy survey done without any professional staff; all the work was done by faculty, undergraduate and graduate students. We are very proud of the success of the project as part of the academic educational process.”

Student members of the Undergraduate ALFALFA Team descend on the Arecibo platform during a tour of the Arecibo telescope.

Koopmann notes that the team includes faculty from disciplines other than astronomy, including physicists specializing in string theory and information scientists. “The students got to know people from all different backgrounds,” she said. “They got to see how research collaboration works and how scientists interact with one another.”

Most of the observing was done via Internet, but each of the faculty members involved spent time at Arecibo, as did many of the students.

“Many of the faculty involved have heavy teaching loads and don’t have the time or resources at their schools to do research,” said Haynes. “ALFALFA gave them the support and the training to engage them and their students in a prominent science project.”

The students helped both with observations as well as data analysis; graduate students also developed software and algorithms for the project. Luke Leisman Ph.D. '17, assistant professor at Valparaiso University, was one of the graduate students involved in the project. “ALFALFA has inspired generations of students,” he said. “ALFALFA, under Martha and Riccardo's leadership, showed me what it meant to do good, careful science, and also how to use that science to make a difference in people's lives.”

Sabrina Stierwalt Ph.D. '09, research associate, Caltech, participated in the strategic planning for the design of the survey, made some of the initial observations, and was among the first to publish ALFALFA-related papers. “It was a grad student's dream to be able to use this incredibly impressive telescope and also to be the first to see the hydrogen gas view of the sky that ALFALFA gives us,” she said. “The exciting science from ALFALFA, and the passion that Martha and Riccardo have for creative and new approaches to studying the universe, are the reasons I'm a scientist.”

Cornell graduate student Mike Jones (PhD '16; left) and Alison Farrish ('16; center) monitor observations with other members of the Undergraduate ALFALFA Team in the control room at Arecibo.

Kelly Hess ’05, one of the undergraduates who worked on ALFALFA and now a postdoctoral researcher at the University of Groningen, said that “everything I’ve learned about the best practices for running a large survey and international collaboration, I’ve learned through my experience with ALFALFA, and I take those skills and experience with me to any new project I approach.”

The Undergraduate ALFALFA Team’s latest project is the Arecibo Pisces-Perseus Supercluster Survey, which builds on ALFAFA data to measure the mass of Pisces-Perseus Supercluster for the first time.

“ALFALFA is the source of many research projects and the seed of many future discoveries to come,” said Koopmann. “It’s a gift to the community.”

The ALFALFA team at Cornell has been supported by the National Science Foundation and by grants from the Brinson Foundation.